Thread: I don't understand this code

I have the following code for binary heaps:

Code:

#define Parent(x) ((x - 1) / 2)
#define Left(x) ((2 * x) + 1)
#define Right(x) ((2 * x) + 2)

#define MAX 10

void swapElem(int *a, int *b) {
	int tmp;

	tmp = *a;
	*a = *b;
	*b = tmp;
}

void insertHeap(Heap *h, int elem) {
	if(h->size == MAX) return;
	
	h->elem[h->size] = elem;
	h->size++;
	
	int idx = h->size - 1;
	
	while(idx && h->elem[Parent(idx)] > h->elem[idx]) {
		swapElem(h->elem + Parent(idx), h->elem + idx);
		
		idx = Parent(idx);
	}
}

But I don't understand the line:

Code:

swapElem(h->elem + Parent(idx), h->elem + idx);

I mean, h->elem is an array, how come h->elem + idx (for instance), produce a number? I'm adding idx to h->elem but h->elem is not a value it's an array.

What does it mean?

Originally Posted by Nazgulled

I mean, h->elem is an array, how come h->elem + idx (for instance), produce a number?

It doesn't. It produces a pointer. A pointer is what the swap() function expects, so all is well.

If a is an array and i is an offset, then the expression a + i is equivalent to &a[i]. Does that make it clear?

Yes, but I think I'll change my code to the expression &a[i], I'm more comfortable with it and I already know what it means. The way I currently have, confuses me a bit.

Thank you.